Extended surface metal heat-exchangers are known. For example, a radiator of a car or a home air conditioner unit embody such heat-exchangers. Extended surface metal heat-exchangers are used because the heat transfer coefficient on the gas side is low, so the area exposed on the gas side is increased. Recuperator tubes of mullite or silicon carbide are used at high temperatures to recover heat because there have been catastrophic failures of metal tubes due to high temperatures. Silicon carbide conducts heat better than mullite, but silicon carbide is brittle and expensive and has to be encapsulated with an oxygen resistant material when used in an oxygen atmosphere at high temperatures. It would be desirable if an extended area heat-exchanger of mullite could be made. It would be even more desirable if the heat-transfer characteristics of silicon carbide could be used; and if the brittleness of the silicon carbide could be overcome, and if the unit could be coated with a metal or a catalyst. Hague International Co. of South Portland, Maine makes a silicon carbide recuperator tube 4 foot long and 4 inches in outside diameter with fins, but the outside area is only 9.8 square feet and the inside area only 3.14 square feet and, generally speaking, it is of limited dimension. The fins are parallel to the gas flow and promote streamline flow instead of turbulent flow. It would be most desirable if these areas could be substantially increased since the heat transferred is proportional to the area. Because of the temperature limitations on the bonding agent for the silicon carbide it would also be desirable to have an extended area heat-exchanger that would operate at high temperatures, and that would promote turbulent flow instead of streamline.
When a ceramic based catalyst is put inside a pipe, as in an auto catalytic converter, it is very desirable to be able to heat it up quickly, because the catalyst is not effective at low temperatures when the car is first started. U.S. Pat. No. 3,768,982 of Kitzner describes a heater but heat must pass through a barrier to heat the catalyst. Parallel hole monoliths are hard to heat because the heat must pass through the ceramic, even when coated with a metal, but in a sponge, heat is conducted via conduction in a zig-zag fashion. A silicon carbide insert in a parallel hole monolith would block holes, but in a random hole sponge, the silicon carbide is easily by-passed. If a hole in a parallel hole monolith becomes plugged with carbon, coal or soot from combustion products, or MMT (methylcyclopentadienyl) manganese tricarbonyl, a fuel additive used to increase gasoline octane) the entire length of the hole is blocked; in a random hole sponge, any block is easily by-passed. Likewise, in an exothermic reaction like methanation, heat can be removed more efficiently in a random hole sponge because the hot gases can contact the inside of the pipe. In an endothermic reaction like reforming it is necessary to put heat into the reaction, instead of removing heat. Uniformity of temperature promotes specificity, i.e., only the reaction desired takes place instead of another reaction at a higher temperature. Heat transfer with pellets is poor because of point-to-point contact.
In a ceramic heat wheel with a catalyst, it is necessary to coat the entire wheel with an expensive catalyst such as platinum.
Glass tube and shell heat exchangers are corrosion resistant but have relatively poor heat transfer coefficients.